It was Christmas 2000, the year of the floods. With the railway line submerged at Machynlleth, I had to complete the journey to my parents' farm by bus. The driver told me how the weather had changed: long-lasting snowfalls were a thing of the past, yet monsoon-style winter rain was getting more frequent. My father and I got drenched as we strung up sheep fencing around a field the following day.

That evening, we watched old family slides of us all in Peru more than 20 years ago, where my father was working as a geologist in the Andes. There was one of me wearing a sombrero, against the backdrop of a vertical ice wall near the town of Huaraz. Best of all were my father's pictures of his work site, far away on the remote eastern side of the Cordillera Blanca (Peru's highest range).

Although the slides were fading, the snowfields of 1980 still gleamed in the tropical sun. The projector whirred again, revealing an enormous fan-shaped glacier looming over a small lake. Icebergs were floating in it, having tumbled down from the glacier above. It was a spectacular sight.

My father narrated: "It was incredibly hard work, carrying drilling equipment around to take rock samples, and then spending freezing nights in those crappy old orange tents. But I loved it." I gazed at the screen. "It may not be the same now," I replied. "I've heard that glacial retreat in the tropical Andes is pretty rapid." My father wasn't so sure. "Maybe it has changed. I don't suppose I'll ever go back, but I wonder what it does look like now."

I was silent for a while. He'd just given me an idea. So began my global journey cataloguing the impacts of climate change across five continents over three years. I paddled in floodwaters on the drowning island of Tuvalu, got battered by dust storms in Inner Mongolia, endured a near hurricane on the Outer Banks of North Carolina and visited disappearing Inuit villages in Alaska. But the high point was to come in Peru, where I arrived in June 2002, armed with a copy of my father's 1980 glacier photo and - with my climbing partner Tim Helweg-Larsen - took the bus up into the mountains to begin retracing his steps.

I was under no illusions. I knew there would be less glacial ice left now than when my father was there. The evidence for rapid glacial retreat - driven by man-made global warming - is overwhelming, and now comes from just about every major mountain range on the planet.

In the European Alps, about half the total glacier mass disappeared between 1850 and 1990. In the US, the original 150 glaciers within Glacier National Park are now down to a mere 50 - and most of these are now tiny.

Somewhere between 500 and 1,000 cubic kilometres of water have melted from Chile's Patagonian ice caps in the last 50 years - enough to add a couple of millimetres to global sea levels. And in the Himalayas, the mighty Rongbuk glacier, which surges down the north face of Everest, retreated between 170 and 270 metres between 1966 and 1997. The only glaciers gaining mass anywhere in the world are in wet maritime areas such as southern Norway, where melting has been offset by increased snowfall - again linked with climate change.

Because of the dynamics of equatorial seasons, tropical glaciers have been worst affected of all. Mount Kenya's icefields lost three-quarters of their entire extent during the 20th century. Kilimanjaro is losing its famous snows at an accelerating rate: this huge volcano's glaciers declined by 80% between 1912 and 2000. The mountain is likely to be ice-free by 2015. There are now less than three square kilometres of ice left on the West Papuan mountains: down from 20 square kilometres 150 years ago.

Meanwhile, back in Peru, which holds the vast majority of tropical glacier mass, rapid rates of melting have also been documented. The Cordillera Blanca is estimated to have lost 15% of its area in less than three decades, while other, smaller ranges have lost nearly a half of their glacier area. And the rate of glacial retreat has been speeding up dramatically since 1980 - it is now three times faster.

Despite all this scientific evidence, I still wasn't prepared for the sight that confronted me when I finally reached the spot where my father had stood back in 1980. The trek up Jacabamba valley had been hard going, even with donkeys to carry most of our load on the lower path. But the day had dawned bright and sunny above the grassy meadow where we pitched the tent; just a few streaks of cloud twisted themselves around the immense, 6,000-metre snowy peaks that loomed at the head of the valley.

The walk up had been surprisingly easy, over mounds of old moraine and across rubble-strewn riverbeds, up the left-hand fork from the top of the main Jacabamba valley. There was little vegetation, just a few scrubby bushes on the slopes higher up, and some intrepid little blue flowers among the boulders. Nor was there any sign of a path. It seemed incredible that my father's expedition had made the trek with heavy loads of geological equipment. When we reached the lip of an old terminal moraine about three hours above our campsite, I stopped again, with a flutter of trepidation. I knew that the answer to my father's question lay just a few metres beyond.

Then I saw. For the first few seconds I couldn't speak. Then I shouted to Tim. "It's gone!" He hadn't even realised we were in the same place, and studied the old photo. "Bloody hell. That's unbelievable." I could scarcely believe my eyes either. The lake was the same - if rather larger, probably because of the extra meltwater - and the rock walls encircling the whole arena just as sheer: but the mass of ice and snow that previously dominated the scene had melted completely away, leaving a few mounds of dark-red grit, streaked white with recently-fallen avalanche debris.

The main glacier was still visible high above, but it had also thinned enormously. It took a while for the magnitude of our discovery to sink in. Tim took some new photos - from exactly the same vantage point as my father - while I sat and tried to comprehend the scene. I had expected the glacier to be smaller, perhaps for it to have even retreated from the lake, but its complete disappearance was a hammer blow. This glacier had survived for thousands of years, only to disappear within a single generation because of human greed and folly.

Back in Wales, I loaded my own new slides of Jacabamba into the projector with a sense of foreboding. First, I showed my father the picture he had taken back in 1980. His eyes sparkled at the recollection. Then I forwarded to Tim's photo, snapped two weeks previously - of the same spot, with just bare rock and the glacier gone. "Good God!" My father leant forward. "I can't believe it. It's so sad. That was the whole character of the place. It was such a beautiful glacier, I've never forgotten it."

To help us grasp the reality of the scene, we flicked back to the old slide, and then back again to the new one. My father still couldn't quite believe it. The reality of the scene etched in his memory had passed away, and the world was the poorer for it.

Today, the only remaining trace of the Jacabamba glacier is a single fading impression on a strip of celluloid, and the similarly fading recollections of a small group of geologists. My father leant back again in his chair, and the sparkle in his eyes was gone. "It's so sad," he said again.

· Mark Lynas is author of High Tide: News from a Warming World, published by Flamingo. Visit www.marklynas.org